Washers and thickeners are relatively expensive components of conventional pulp mills. Conventional thickeners and washers usually comprise a cylindrical drum to which a vacuum is applied over the forming and washing zones. The rate of application of the pulp as a sheet to the rotating vacuum drum exterior surface is typically slow, on the order of about 1 meter per second for conventional pressure washers. The drum also rotates relatively slowly, on the order of about 4 rpm. Therefore, the amount of pulp treated per screen area is relatively small. Since the cost of conventional thickeners/washers is strongly dependent upon their size, such devices typically end up being expensive components of the mills.
It has been recognized that the thinner the layer of pulp through which the water must travel, the faster the thickening and washing operations can be accomplished, and the smaller the thickening/washing machine may be. However the art has not been able to make practical use of this known physical phenomena.
One way that use of the thin sheet advantages can be achieved is by utilizing centrifugal force to cause the filtrate to move through the pulp. In U.S. Pat. No. 3,289,843, an apparatus is described having several different embodiments for centrifugal washing/filtering. However such centrifuges have not proven particularly effective in the pulp and paper field.
According to the present invention, a method and apparatus are provided that take advantage of centrifugal force to cause thin sheets of pulp to be formed on a screen, and also provide the motive force for moving the filtrate through the pulp. Utilizing the invention, it is possible that washers having about a 10 times smaller screen area compared to conventional vacuum drum washers are possible, thickening and washing can be obtained without any rubbing seals to confine a vacuum or the like, there is no necessity for external devices such as droplegs, vacuum pumps, or fans, to provide the motive force for water to move through the pulp.
According to one aspect of the present invention, an advantageous pulp thickener/washer is provided which comprises the following elements: (a) A perforated cylinder having an interior and an exterior. (b) Means for applying pulp in a thin sheet on the interior of the cylinder at a first arcuate position thereof. (c) Means for rotating the cylinder about an axis of rotation so that liquid in the sheet is caused to move radially outwardly through the cylinder as a result of centrifugal force, so that the pulp is thickened. (d) Means for withdrawing the pulp from the interior of the cylinder at a second arcuate position, less than 360.degree. from the first position. (e) Means for moving the withdrawn liquid and treated pulp away from the thickener/washer for subsequent treatment, use, or disposal. And, (f) means for applying wash water to the pulp at the interior of the cylinder at a third arcuate position between the first and second positions.
The means (b) preferably comprises a stock head box similar to that utilized to form sheets of paper or pulp. The velocity of pulp from the head box, which is provided by line pressure, is preferably at least about 10 meters per second, and typically at least about 20 meters per second, to approximately match the speed of the rotating cylinder. The means (f) also comprises a wash water head box. The means (d) preferably comprises a vacuum roll, although an air doctor or steam doctor alternatively may be utilized. The means (e) can be selected from a wide variety of chutes, conveyors, or the like, but preferably comprises a screw conveyor extending in a dimension generally parallel to the axis of rotation of the screen cylinder. The means (e) for filtrate removal preferably comprises means defining channels in a drum surrounding the screen, an impeller, and a volute, with a cut-off shoe for the impeller at an arcuate section generally between the first and second arcuate positions, to facilitate pulp removal at that point. The means (c) preferably comprises a drive roller which operatively engages the exterior of the cylinder to rotate it about a generally horizontal axis, and the cylinder may be a cylindrical perforated plate, or a cylindrical wire cloth.
According to another aspect of the present invention, a method of acting on paper pulp to thicken and/or wash it utilizing centrifugal force is provided. The effective method according to the invention comprises the steps of substantially sequentially: (a) Applying pulp in a thin sheet on an interior of the cylinder at a first arcuate position. (b) Rotating the cylinder about its axis to cause liquid in the pulp to move radially outwardly, under the influence of centrifugal force, through the cylinder, to be discharged. (c) Withdrawing the sheet from the cylinder at a second arcuate position less than 360.degree. from the first position. (d) Passing the pulp to a further treatment or use stage. And, (e) adding wash water to the interior of the sheet between the first and second arcuate positions. Step (a) is practiced at a rate of at least about 10 meters per second (e.g. about 20 meters per second), and step (b) is practiced at a rate of at least about 25 rpm (e.g. about 100 rpm). Paper pulps entering at a wide variety of consistencies may be treated, and will typically have a consistency of about 8-12% during and/or after treatment.
It is the primary object of the present invention to provide for the effective washing and/or thickening of paper pulp or like suspensions, utilizing centrifugal force as the main motive force for filtrate movement. Practicing the invention it is possible to reduce the screen area/daily ton of pulp processed, thereby significantly decreasing washer costs, while at the same time providing effective treatment. These and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.